Elasticsearch 5.0 can read indices created in version 2.0 or above. An
Elasticsearch 5.0 node will not start in the presence of indices created in a
version of Elasticsearch before 2.0.

Reindex indices from Elasticseach 1.x or before

Indices created in Elasticsearch 1.x or before will need to be reindexed with
Elasticsearch 2.x or 5.x in order to be readable by Elasticsearch 5.x. It is not
sufficient to use the upgrade API. See Reindex to upgrade for more details.

The first time Elasticsearch 5.0 starts, it will automatically rename index
folders to use the index UUID instead of the index name. If you are using
shadow replicas with shared data folders, first
start a single node with access to all data folders, and let it rename all
index folders before starting other nodes in the cluster.